There have been suggestions that Benedict Cumberbatch has of late lost all his mirth. First, after one of his nightly post-Hamlet appeals to help Syrian refugees, he called on the audience to “f*** the politicians”. Then he enhanced the impression that he was taking himself a touch too seriously by calling for a meeting with the home secretary about the refugee crisis.

And now, as we discuss the Sherlock Christmas special, The Abominable Bride, he objects to the use of “bromance” to describe the relationship between his Holmes and Martin Freeman’s Dr John Watson.

We are sitting in a shabby room, heated by a small electric fire, in the Bottle Yard Studios, a former factory in Bristol where the set has been built for The Abominable Bride. For the one-off special the creators have abandoned the modern-day setting in which Cumberbatch made his name as Sherlock and taken the world’s most famous detective back to the Victorian era in which Sir Arthur Conan Doyle introduced him.

Cumberbatch is on a break from filming and arrives in Victorian waistcoat and starched collar, his hair slicked back. “What a handsome man,” mutters another (male) journalist as he enters the room. While Freeman — who had been in a few minutes earlier, huddled in an enormous parka — was so low-key that you’d walk past him in the street, Cumberbatch glows like a full moon on a cloudless night.

Of course, we all want to know if Sherlock and John are still the same back in the 19th century. “Is there still the bromance?” I ask.

“You just really want to write the word bromance,” says Cumberbatch to a little peal of sycophantic laughter from some of the journalists gathered around him. The word bromance may be overused in connection with Sherlock, but the nuances of the relationship are a subject of endless fascination to Sherlockologists. I try to keep it light: “There can’t be an article without it in there,” I joke. “There can,” says Cumberbatch. “You can be the first. Strive for change in the press.”

This is starting to feel like it could be hard work, but fortunately Cumberbatch warms up. The essence of the modern-day Sherlock has been maintained in this new departure, he says, so we can expect a rollicking pace, crackling wit and the sparky Holmes-Watson relationship. “It’s a companionship that has evolved in our version, so we are not regressing to ‘Wow!’ or ‘Golly, Holmes!’ or some sort of Nigel Bruce-esque kind of adoration. [It is] more complex than that, which is great.”

This Victorian version will not be a pastiche. “We don’t want to make it into a sketch. We don’t want to make it into something ridiculous or comic,” Cumberbatch says. “We want to be true to the original; at the same time we want to be true to our version. So it’s a very delicate balancing act.”

How did he react when the show’s creators, Steven Moffat and Mark Gatiss, told him that Holmes was going home to 1895? “I was thrilled. I went, ‘At last I can have a f***in’ haircut. [Instead of] that ridiculous bouncy lot of curls on my head.’ And then I went, ‘You are mad. What?’ I genuinely didn’t understand how they were going to get away with it. And then the more detailed pitch came and I went, ‘OK, this is going to be great fun.’ And it really is. It is so nice to play him in his era. Some of the weight is taken off you and [you are]not trying to establish this man in the 21st century. When he is in full Victorian swing it’s a really lovely feeling.”

Playing Holmes in the modern era can sometimes feel like heavy lifting, says Cumberbatch. He is “a man clearly slightly out of his time. To put him back in the era that he’s written in is a joy. It feels easier to a degree. Things that I try to impose a little bit on the modern version, like his physicality, stature, a lot of that is done by the body of the clothing and collars, the deerstalker and cape and all those sort of things. That’s an absolute delight and yet it doesn’t feel like cliché because you are functioning in them rather than quoting them.”

Victorian Sherlock smokes two types of pipe and instead of the Belstaff coat worn by the modern incarnation, he has an inverness cape that echoes the coat, and is worn with matching deerstalker. No doubt they’ll be producing cheap knock-offs in China within hours of transmission.

Gatiss gave Cumberbatch the Belstaff coat after recording the first series and he wore it for a bit before the show was aired. He was not famous then and there was no reason to be concerned about having his picture taken in it. But he worried. “What if someone did accidentally [take a picture] and then says, ‘Seen walking around Hampstead Heath in his f***ing costume.’ Seal my reputation as being a dick.”

Gatiss describes this Victorian Sherlock as “The Adventure of Having Your Cake and Eating It. It is still our show. It is essentially Sherlock as if we have always done it, period. So it hasn’t suddenly become very dusty and slow.”

The 90-minute special is about having fun, chips in Moffat. “Fun for us and fun for the audience. Benedict and Martin are the Holmes and Watson of their age. They own those roles. Wouldn’t it be awful if you never saw them do it properly? Wouldn’t you like to see them in the deerstalker and bowler hat?”

Freeman says he doesn’t feel as though he is playing a completely different character, although “it does tighten you up slightly physically and vocally, I think, so there is slightly less overly casual modern speak”. Once Watson is back in the Victorian era he is a little more in awe of Holmes than in the modern version. “The original Watson was much more outwardly generous in his thinking that his friend is a genius. Modern Watson definitely thinks his friend is a genius but also an enormous pain in the arse.”

The Victorian Watson comes with a magnificent Victorian mustache. Freeman also had a ’tache in series 3 of the modern Sherlock. “I’ve got to try and rein that in, not let Steven and Mark think this is an ongoing thing now and end up as Robinson Crusoe.”

The Abominable Bride is based on a passing reference to a case made in Conan Doyle’s story The Adventure of the Musgrave Ritual. The producers tell us that the story begins with a man seeing his wife in her wedding dress shortly after she killed herself and the trailers suggest a gothic adventure with clattering hansom cabs and spooky locations. No preview tapes have been made available.

On set, the actors and production team give little away, although we are allowed to venture into Holmes’s bachelor flat, which is set up exactly like his slightly squalid pad from the first three Sherlock series but filled with Victorian furniture and paraphernalia. A table is jumbled with scientific experiments, a skull, microscope and typewriter. The bison skull on the wall is still there, but instead of headphones it has an ear trumpet.

Una Stubbs will be there as Holmes’s landlady, Mrs Hudson. The Victorian Mrs H is a little rattier than the modern one, says Stubbs, who adds that Sherlock is the biggest success that she has been involved in during her long career. “I have been lucky to be in some series that have been a success — nothing like this. I am stopped in the street by groups of Russians, Chinese. It is quite extraordinary.” She recalls walking along the street the day after the first Sherlock aired. “I could hear, ‘How do you say it? Benedict Cumber . . ?’ I thought, ‘Oh my God, it’s a success.’ ”

In the subsequent five years, Stubbs has watched Cumberbatch being catapulted to global stardom. “Can you believe how it’s changed? Really incredible. From being this young actor and suddenly this superstar. But it sits well on his shoulders.”

After three Hobbit films Freeman is a similarly big name worldwide. “I was dreading in a way that they would change but they haven’t,” says Stubbs. “You just don’t know if they’d be big-headed because it is a phenomenon, how they have both been drawn up and given all these wonderful opportunities. Well, they’ve earned them,” she adds quickly.

For Freeman, Sherlock is an opportunity to work with his partner, Amanda Abbington, who plays Mrs Watson. These days he is away from Abbington and their two children a lot. “There is no way round it. Well there is — I could stop acting. It would mean not taking interesting opportunities. And even though it’s second in importance to my family, it’s a close second because I was doing it before I met Amanda and before I became a dad. I am very, very passionate about my job but I have to really want to do something to go away.” Work and family life mean that he and Cumberbatch tend to see each other only on Sherlock shoots. “We don’t hang out an awful lot.”

Most of the time he is grateful for the constant flow of work. “I am thinking mainly ‘Thank God’ but with moments of ‘I wish it would ease up.’ You want to have your cake and eat it in life. We are reasonably selfish creatures, I guess. I try not to look at the diary, that way I end up in trouble. Amanda says, ‘Please look at your schedule.’ I say, ‘Yes,’ but it’s boring. I just go where I’m pointed. I can’t think of things too far into the future, my brain does tend to shut down.”

Freeman’s future, he says, will continue to include Sherlock. A new series of three dramas will be filmed next year and is expected to be broadcast in 2017. “I have always believed in doing things as long as one wants to do them and as soon as you don’t want to do something you should stop.” He will work on Sherlock “as long as we are free and enjoying it. I know it’s a good show. The truth is, though, it has got more and more difficult to factor in.”

Gatiss says: “We have accidentally cast the two biggest stars in the country.” Moffatt says: “Everybody else pays them more than we do’

Cumberbatch says he is “pretty determined” to keep playing Sherlock in the modern day and maybe even as a Victorian sleuth in more specials. “I’m still enjoying it and we’ll see how the next series goes. I would love to keep ageing with him. Martin and I started this relatively young compared to a lot of Holmes and Watsons, so why not?”

Benedict Cumberbatch and Martin Freeman are turning back the clock to Sherlock’s origins, finds Tim Martin

In the sitting-room of 221B Baker Street, whereBenedict Cumberbatchand Martin Freeman have jostled and bickered as Holmes and Watson since their modern-day adventures began in 2010, something is different. Watson’s chrome MacBook has been replaced by a clunking manual typewriter. The bison skull on the wall no longer wears its ironic headphones and the view from the window shows a dour set of 19th-century tenements instead of the bustling north London of today.

For the one-off episode of Sherlock, the show’s creators, Steven Moffat and Mark Gatiss, have set their sparkling contemporary adaptation spinning back in time to the era of Sir Arthur Conan Doyle, in a special installment filled with Gothic stylings, gas-lit intrigue, luxuriant mustaches and one very famous deerstalker.

“I thought, at last I can have a f—— haircut. We could get rid of that ridiculous bouncy lot of curls on my head,” says Benedict Cumberbatch, beaming with amusement in a dressing room under the Bottle Yard Studios in Bristol. That wish, at least, came true: today Cumberbatch’s hair is slicked back, Victorian-gent-style, and he wears a stiff collar and dapper tweeds. “Then I went – you’re mad,” he continues. “I genuinely didn’t understand how they were going to get away with it.”

Like his colleagues, he is allowed only to expand on the most uncontroversial Sherlockian topics. A fourth season is coming (“I’ve said we’ll go into production on it in a year, not any specific year,” Gatiss has said) and Cumberbatch deploys his most diplomatic smile when quizzed on how this slice of Victoriana ties into the show’s present-day setting – or how it resolves the desperate cliffhanger at the end of the last season. “Nice try, nice try,” he mutters wryly, conceding only that while the trip into the 19th century “is a great deal of fun, it does advance things. It’s not just on its own.”

He’s far more comfortable when describing how it feels to explore the character as Conan Doyle originally wrote him. “In the sense that he’s a man slightly out of his time,” he says, “to put him back in the era that he’s written in is a joy. Things I tried to impose a little on the modern version – his physicality, his stature – are done by the costumes: the collars, the deerstalkers and cape. It’s a delight.”

The transition to a more polite age, however, hasn’t affected Sherlock’s ruthless bluntness. “He’s a meritocracy,” says Cumberbatch. “He cuts through mediocrity. It doesn’t matter if you’re a lord or a lady, if you’re driving a hansom cab or if you’re one of the Baker Street Boys: it’s purely about what your worth is and your quality, not your social standing. So, yes, he’s still rude. He’s rude to idiots and people who are pompous or sexist. He’s quite a crusader in that regard.”

Freeman, who, as Watson, sports a great caterpillar-like false mustache for this episode, says, “We’re trying not to be too winky about it, and too self-knowing. Sometimes when things are set in, say, the Seventies, it’s our way of laughing at people from the old days. I don’t find that very interesting.”

Instead, he says, the period setting “makes you amend what you’re doing very slightly. The plan wasn’t to suddenly present a whole different character, or a pastiche, but it does tighten you up physically and vocally.”

The surprising tenderness of the Holmes-Watson relationship – the bromance that launched a thousand overheated fan fictions – survives the transition to a starchier age. “I guess the difference is that Victorian Watson’s got more patience,” says Freeman. “He’s much more generous with his outward thinking that his friend’s a genius than modern Watson. Modern Watson definitely thinks his friend is a genius, but he’s also an enormous pain in the a—.”

Upstairs, and back on set, Moffat and Gatiss are pacing around preparing for the next scene. Theirs is a scarcely less fascinating double act than Holmes and Watson’s: Gatiss is willowy, blond and cheerful, Moffat short, dark and brooding, but they finish each other’s sentences with the ease of long friendship and collaboration.

A Victorian episode, Gatiss carefully explains, doesn’t represent a long-held dream, because “that would imply we’d been begrudgingly making it modern to get to this. We’ve joked about the idea for a long time, but it’s just so massively appealing to do both. The only other people who’ve ever done it” – played Sherlock, he means, in both a period and a modern setting – “are Basil Rathbone and Nigel Bruce.” Moffat nods. “The show had to be as big as it is now,” he interjects, “for us to do something this mad.”

In fact, says Moffat, the elaborate Victorian trappings of the set represent the “lazy version” of the screenwriter’s task. “The one we put all the thought into,” he says, “was finding the equivalents for the modern day.” Transposing Cumberbatch and Freeman’s relationship to the past was actually “exactly the same trick as we did a few years ago with [the first Sherlock episode] A Study in Pink. We said, look, it’s still Sherlock Holmes, isn’t it? Even though we’ve changed everything.”

Around the corner in the editing suite, a quick glimpse of a rough cut proves that beneath its period trappings this is, recognisably, the same Sherlock. Strand Magazine in hand, a harried-looking Watson is trying to defend himself against slights on the veracity of his casebook from his landlady Mrs Hudson, played by Una Stubbs. “According to you I just show people up the stairs and serve you breakfast,” she complains. “I’m your landlady, not a plot device!”

“You’re not the only one, Mrs Hudson,” remarks Cumberbatch’s Sherlock. “I was hardly in the one about the dog.”

Nearby is Amanda Abbington, Freeman’s wife both on and off-screen who, at the end of the last season, was shown to be an assassin with a deadly past. “We’ve kept Mary’s sparkiness and her glint,” she says firmly. “She’s got a bit of scrappiness about her and we don’t want to lose that.”

She laughs when asked if she and Freeman enforce a no-Sherlock rule in their household. “Definitely not!” she says. “We sit together as a family and watch it. My son knows a few of the scenes and does them with my daughter. Grace is usually the assassin shooting Joe. But, no, there’s no embargo on Sherlock. We love it!”

As Cumberbatch and Freeman prepare to return to set, they mutter about the show’s excursion into period drama. Freeman laments the fact that he can’t get dressed by himself, while Cumberbatch has period arcana to deal with, including a fancy meerschaum that is, he notes darkly, “a pyrotechnic pipe. That doesn’t mean it blows up in my face. I’m just not smoking.”

It is widely believed that the fourth season of Sherlock may be its last, and although Gatiss cheerily acknowledges that “making people wait is all we do: whole civilizations have risen and fallen between seasons of Sherlock”, the snowballing fame of its actors has made synchronizing diaries more and more difficult. Neither of the two leads seem worried, however. “I’ve always believed in doing things for as long as one wants to do them,” says Freeman.

And if that sounds ever-so-slightly cryptic, Cumberbatch is more definite. “I don’t know any other actor that’s been so spoilt with this role,” he says. “I’ve said many times I’d love to keep aging with him. And Martin and I started this fairly young compared to a lot of Holmes and Watson pairings. So why not?”